1. Field of the Invention
The present invention relates to a battery charging circuit and an electronic device including the same, and more specifically, relates to a battery charging circuit such as for a lithium ion battery (hereinbelow called as LI battery) which is adapted to detect a condition of over current charging and a condition of charging completion by making use of a microprocessor and an A/D converter allowing a large charging current even when a converting bit number of the A/D converter is small.
2. Description of Related Art
In electronic devices such as office automation (OA) devices including a lap-top-computer and audio visual (AV) devices including a video camera, a LI battery is loaded for the purpose of a battery back-up or a battery drive. The LI battery is charged either when the output is lowered due to its electron charge discharge or periodically.
Generally, a constant voltage and constant current power source is used for charging such as the LI battery and when the battery restores to a predetermined voltage via charging, the charging current gradually decreases depending upon progress of charging. In such charging operation, there are two cases, one, the charging is started from a condition that the substantial electric charges in the battery have been discharged, the other from a condition that little electric charges in the battery have been discharged. In the former case, the charging is performed at first by flowing a constant current of about 1A. In the latter case or in a condition wherein a charging operation has advanced to a certain extent, the charging is performed at a constant voltage other than at a constant current in relation to the restored voltage of the battery under charging and in such instance the charging current gradually decreases. Under the constant voltage charging, when the charging current reduces, for example, below about 5 mA or when the charging current reduces, for example, blow about 300 mA and the charging time under the constant current charging exceeds a predetermined time, it is judged that a sufficient charging is performed for the battery and the charging operation is terminated. Then completion of charging is indicated to notice the termination of the charging.
FIG. 6 shows one example of conventional battery charging circuits performing the above explained control.
A controller 1 includes such as a microcomputer and, at the start of charging operation, sends out a control signal A to a regulator 2. For the regulator 2 a constant voltage constant current power source circuit is usually used and the regulator 2 begins a charging operation in response to the control signal A and carries out either a constant current charging operation or a constant voltage charging operation and a charging current B is supplied to a battery 3 such as a LI battery via a diode D.sub.o.
The charging current B also flows through a resistor R inserted between the battery 3 and a grounding line. The resistor R constitutes a current-voltage converting circuit 4 which converts the value of the charging current B into a voltage signal C. The voltage signal C is applied to an A/D converter (A/D) 5 and is converted there into a digital signal D and inputted to the controller 1. Herein, the resistor R1 and the A/D 5 constitute a detecting circuit of the charging current value for the controller 1.
The controller 1 monitors the magnitude of the charging current B based on the amount of the digital signal D and terminates the transmission of the control signal A when the amount of the digital signal reduces below a predetermined value representing completion of charging. As a result, the regulator 2 terminates the supply of the charging current B. Further, the controller I operates as a protection circuit for detecting an over current charging, in that, when an over current exceeding a predetermined amount is detected through a continuous monitoring of a signal inputted from the A/D 5 during charging operation, the control signal A is ceased and the operation of the regulator is terminated.
This protecting operation is indispensable for performing a safe charging operation with these kinds of charging circuits. For fulfilling the above demand, the controller including such as the microcomputer necessitates an A/D converter which convertes a current value during the charging operation into a digital value.
A charging current detecting range with the controller 1 is usually from a few mA to a maximum of about 1 A. For this reason, in the above indicated condition the A/D converter is required to have a resolution below 5 mA and a convertible range equal to or more than 1 A, and for meeting this requirement an A/D converter having equal to or more than 8 bits is required.
However, recently, a demand of a high speed charging is increasing as well as an upper limit of charging current tends to exceed over 1 A. For coping these demands and in view of an over current charging detection, the bit number of the A/D converter has to be further increased, thereby an A/D converter having 10.about.16 bits is necessitated.
However, a cost of a high accuracy A/D converter having many bit number is high, so that such A/D converter is hardly used for a charging circuit having a severe cost limitation. Further, the internal wiring lines thereof increase in accordance with the bit number increase and such is insuitable for a battery charging circuit used in electronic devices such as portable electronic devices having an intense down-sizing requirement. Still further, when the circuit is formed into an IC, an integration of other circuits is made difficult in proportion to the increase of the wiring lines.